Sequential mate encounters: female but not male body size influences female remating behavior

Whether and how individuals choose sequentially among matesis an important but largely neglected aspect in sexual selectionstudies. Here, we explore female remating behavior in the cellarspider Pholcus phalangioides. We focus on body size as one ofthe most important traits involved in mate choice. Large andsmall females (n = 216) were double mated with large or smallmales in all eight possible combinations. All females copulatedwhen virgin, but only 82% accepted a second male. The chanceof a female remating was not significantly predicted by thebody size of the second or first male or by the size differencebetween the two. In contrast, a previous study demonstrateda male size effect in that larger males monopolized femalesuntil egg laying when two males of different sizes were present.We suggest that sequential encounters are more common undernatural conditions than male monopolization of females becauseestimates of concurrent multiple paternity together with observationsin a natural population do not favor mate guarding as the predominantmating strategy in this species. It follows from our study thatthe intensity of sexual selection on male size may be greatlyoverestimated when using a competitive laboratory setting fora species in which females generally encounter mates in a sequentialfashion. Female remating probability was significantly predictedby female size, with large females remating with higher probabilitythan small females. Thus, when mating with large females, malesmay gain higher fertilization success through increased femalefecundity but also face a higher sperm competition risk.     

Sexual selection for large male size in a polyandrous butterfly: the effect of body size on male versus female reproductive success in Pieris napi

Sexual selection theory predicts that the larger sex shouldbe that for which fitness increases at the faster rate withsize. In butterflies, as in most invertebrates, females areusually the larger sex, but previous comparative analysis hasshown that relative male size increases with female polyandryamong butterflies. In agreement with this pattern, males arelarger than females in the strongly polyandrous green-veinedwhite butterfly, Pieris napi L., and in this article we assessthe size dependence of reproductive success in both sexes. Inan experiment where virgin males and females were released inthe field, we found no strong association between size and malemating success. However, laboratory experiments showed thatthere was a strong correlation between size and the ejaculatethat the male delivered to the female at mating and that largeejaculates delayed female remating for a longer time comparedto small ejaculates. Moreover, female P. napi utilize male-derivednutrients received at mating to increase their fecundity. Hence,large males sire more offspring both by way of donating morenutrients to female egg production and by way of delaying femaleremating (given that the last male to mate with the female willfather most of the offspring). Laboratory experiments showedthat the association between size and fecundity was low, ornonexistent, among P. napi females allowed to mate only once.However, weak size dependence was found for polyandrous females.We hypothesize that size dependence of female fecundity maybe especially weak among polyandrous butterflies because a fundamentalsource of variation in fecundity relates to their ability tofind nutrient giving males, an ability which may be unrelatedto female size. According to this hypothesis there is a causalassociation between weak size dependence of female fecundityand polyandry, and a strong size dependence of male reproductivesuccess that may underlie the comparative pattern of positivecorrelation between relative male size and polyandry.     

The influence of male and female body size on copulation duration and fecundity in Drosophila melanogaster

We studied two components of the mating system, copulation duration and early fecundity, in relation to body size in Drosophila melanogaster. Body size variation was created experimentally by varying the degree of crowding (starvation) among larvae from an inbred strain, keeping the genetics and temperature as constant as possible. Hence, in contrast to most previous studies, where genetic and environmental variation have been confounded, we aimed at investigating how much pure phenotypic variation could influence copulation duration and early fecundity. It is shown that copulation duration and fecundity both strongly dependent on female body size, but either not or much less so on male body size. Small females mate faster than medium or large females and small females have the lowest fecundity. Among males, medium-size males are more fecund than smaller or larger males, resulting in stabilising selection for intermediate male size. These results are in contrast with previous findings.     

The relationship between female body size and egg size in pipefishes

Comparing five species of pipefish, egg size was significantly larger in species with brood pouches (Syngnathus typhle, Syngnathus acus and Syngnathus rostellatus) than in species without brood pouches (Entelurus aequoreus and Nerophis ophidion). Egg size correlated positively with female body size in species with brood pouches, but was similar across female sizes in the species lacking pouches. These results may reflect differences in offspring competition as a consequence of variable offspring relatedness within a brood, due to the mating systems adopted by the different species and the presence or absence of a brood pouch.     

Field evidence that host selection by conopid parasitoids is related to host body size

Summary The body size of the host insect in which a parasitoid develops can have important effects on its development and life history. Large and small host body size have both been suggested to be advantageous to parasitoids, depending on the life-history of the species concerned. We test field data on the bumblebeeBombus terrestris and its conopid parasitoids for evidence of differences in size between parasitised and unparasitised worker bees. Bees acting as hosts for conopid parasitoids are on average larger-bodied than unparasitised bees. This result holds for bees collected in two different years, and whether bees are collected while foraging or from the nest. The results we present demonstrate differential parasitism of hosts of different body sizes, but do not necessarily indicate active host choice by conopids. However, they are in agreement with independent evidence that conopids develop more successfully in large-than in small-bodied hosts.     

The effect of female body size on male mounting behaviour in Dermacentor variabilis and D. andersoni

Abstract. This study was undertaken to determine if body size of female ticks was an important factor in eliciting the mating behaviour of Dermacentor variabilis and D. andersoni males. Dummy female ticks (DFTs) representing varying sizes of feeding females were prepared from plastic beads. Size of DFT was shown to be important in determining both the time spent by males in contact with DFTs and the extent of male mating response released. Dermacentor varlabilis males preferred the smaller-sized DFT, whereas D. andersoni males preferred the larger sizes. Males of both species were able to discriminate between size of DFT only when mounting sex pheromone (MSP) was present.
Size of the DFT was more important than the composition of the MSP extract in determining the time males spent in contact with DFTs.
Males of both species were sensitive to variations in concentration of extract, and spent 2–6 times longer in contact with DFTs when the concentration was optimal than when it was not. No significant difference in the male's mating response was seen in response to variation in concentration of heterospecific extract.
Male mating response, i.e. the behaviour of the male as it progresses through the initial contact, climbing onto the dorsal surface and turning onto the venter, was shown not to be dependent upon how long the male spent in contact with the DFT. Male D. andersoni had a much lower mating response to DFTs than D. variabilis males, suggesting that some further stimulus may be required.     

Empirical evidence for fast temperature-dependent body size evolution in rotifers

Organisms tend to decrease in size with increasing temperature by phenotypic plasticity (the temperature-size rule; ectotherms) and/or genetically (Bergmann’s rule; all organisms). In this study, the evolutionary response of body size to temperature was examined in the cyclically parthenogenetic rotifer Brachionus plicatilis. Our aim was to investigate whether this species, already known to decrease in size with increasing temperature by phenotypic plasticity, presents a similar pattern at the genetic level. We exposed a multiclonal mixture of B. plicatilis to experimental evolution at low and high temperature and monitored body size weekly. Within a month, we observed a smaller size at higher temperature, as compared to body size at lower temperature. The pattern was consistent for the size of both mature females and eggs; rotifers kept at high temperature evolved to be on average 14% (after 2 weeks) and 3% (after 3 weeks) smaller than the ones kept at low temperature (10 and 5% in the case of eggs, respectively). We therefore found that B. plicatilis is genetically programmed to adjust its body size-to-environmental temperature.     

Seasonal trends in body size of adult male mosquitofish,Gambusia affinis,with evidence for their social control

Synopsis Collections from a natural population of mosquitofish,Gambusia affinis, in southern Indiana showed that males born early in the breeding season mature rapidly at a small body size, while males born later in the breeding season delay maturation and achieve larger body sizes. A field experiment, involving removal of males from pond populations, was conducted to test the hypothesis that delayed maturation by late summer males is, at least in part, under social control. Mean total length (TL) of adult and maturing males in late summer in ponds from which large numbers of males had previously been removed was significantly lower than mean TL of males in control ponds. These data support the hypothesis of social control and support Sohn's competition-predation hypothesis for control of adult male body size inGambusia.     

Male contest investment changes with male body size but not female quality in the spider Nephila clavipes

Animals use rules to adjust their level of investment in a contest. We evaluate male strategies during contests over females in the golden orb-web spider Nephila clavipes. We tested whether male behaviour changes with female value, and found that contests were similar in intensity and outcome whether the female was a juvenile or adult, virgin or non-virgin, or whether one male had invested sperm in the female. We found evidence that males use a self-assessment strategy when deciding to withdraw from a contest. Loser body size and contestant size difference were correlated with a higher frequency of contest escalation, and fights involving two large males were more likely to escalate than a fight in which one male was small. A multiple regression showed that loser body size had a stronger effect on contest escalation than contestant body size difference. More importantly, the size of the winning male had no effect on contest escalation, a key prediction of a self-assessment strategy. In N. clavipes, body size is the primary factor that determines the outcome of male contests, and males do not appear to assess their opponent or the quality of the resource when deciding to withdraw from the fight.     

Sex-chromosome pairing: evidence that the behavior of the pseudoautosomal region differs during male and female meiosis.

In humans, recombination in the pseudoautosomal region is approximately 10-fold higher in males than in females. This difference is thought to reflect the fact that, in females, there is opportunity for genetic exchange along the entire length of the X chromosome, resulting in a relative reduction in the likelihood of exchange in the pseudoautosomal region. In two instances in the laboratory mouse where X-chromosome pairing and exchange in females are limited to the pseudoautosomal region, a significant level of X-chromosome pairing failure was observed at diakinesis/metaphase I. Further analysis indicated that, in female meiosis, the inability of the X chromosome to consistently form a pairing configuration via the pseudoautosomal region alone is not a property of the pseudoautosomal region per se but is due to the fact that it resides on an X chromosome. Thus previously reported sex-linked differences in recombination rate in the pseudoautosomal region may actually reflect differences in pairing and/or recombination of the pseudoautosomal region on an X chromosome undergoing male versus female meiosis.     

Effects of male size and female dispersion on male mate-locating success in Nephila clavipes

The reproductive success of sexually reproducing organisms depends on their capacity to locate potential partners. Research on the mating systems of web-building spiders, like other terrestrial invertebrates, has focused around male courtship and the signalling behaviours that facilitate copulation. In contrast, both mate searching and mate location behaviours are largely ignored and rarely quantified in field studies. We conducted a series of field removal experiments to explore the effects of male body size and female dispersion on mate locating behaviour of the golden orb-weaving spider (Nephila clavipes). The number of males that arrived on females’ webs was influenced by travel distance, male body size, male body weight and female dispersal. More males returned to webs that were closer to the release point; smaller males experienced a greater weight loss; and larger numbers of males arrived at webs where males had been previously present. Female dispersion influenced the size but not the number of arriving males: males were no more likely to arrive at solitary than aggregations of webs, but males arriving at solitary webs were typically larger than those arriving at aggregations. We discuss the implications of various male traits that could act as selective pressures on male mate location and mate searching behaviours.     

Prior intrauterine position influences body weight in male and female mice

In two longitudinal studies, intrauterine location of male and female Rockland-Swiss mice relative to fetuses of the same and opposite sex dramatically influenced body weight. In one study, body weight of males and females that were located in utero between two female fetuses (OM animals) or between two male fetuses (2M animals) was assessed from birth to the time of weaning (25 days of age). The body weights of 2M females were indistinguishable from those of OM and 2M males on all but a few of the 25 postnatal days of life. Also, 2M females were significantly heavier than OM females from Postnatal Day 6 onward, and 2M males weighed significantly more than OM males from Postnatal Day 19 onward. In a second study, food intake and body weight of animals from different intrauterine locations were examined from 25 to 120 days of age. Regardless of prior intrauterine position, males were always heavier than females. However, prior in utero location modulated body weight in that 2M females were significantly heavier than OM females and 2M males were reliably heavier than OM males. Intrauterine position effects were observed in the absence of any appreciable influence of this variable on levels of food intake. Taken together, the results suggest that prior in utero location may influence metabolic set points involved in the regulation of body weight and fat storage.     

Effects of female and male size on female mating and remating decisions in a bean beetle

The body sizes of individuals of the choosing and chosen sexes in a mate choice may affect sequential mating of females. We examined the effects of the body sizes of females and their mates on attributes of female first mating, and the effects of body sizes of females and their previous and potential future mates on female remating in the adzuki bean beetle, Callosobruchus chinensis. Large- and small-sized adults were derived from larvae reared under conditions of low and high density in a bean, respectively. The speed of first mating of large females was not affected by the size of courting males, whereas small females initiated mating more rapidly when courted by small males. The remating probability of large females was not affected by first male size, whereas small females that mated first with smaller males were more likely to remate. These data suggest that pre- and post-copulatory female choices for male size depend on the female’s size, and the small females might be more willing to copulate with smaller males but prefer larger males to sire their offspring after copulation. A possible explanation for this preference is that small females may suffer greater harm from copulating with larger males.     

Intraspecific evidence from guppies for correlated patterns of male and female genital trait diversification

The role of sexual selection in fuelling genital evolution is becoming increasingly apparent from comparative studies revealing interspecific divergence in male genitalia and evolutionary associations between male and female genital traits. Despite this, we know little about intraspecific variance in male genital morphology, or how male and female reproductive traits covary among divergent populations. Here we address both topics using natural populations of the guppy, Poecilia reticulata, a livebearing fish that exhibits divergent patterns of male sexual behaviour among populations. Initially, we performed a series of mating trials on a single population to examine the relationship between the morphology of the male's copulatory organ (the gonopodium) and the success of forced matings. Using a combination of linear measurements and geometric morphometrics, we found that variation in the length and shape of the gonopodium predicted the success of forced matings in terms of the rate of genital contacts and insemination success, respectively. We then looked for geographical divergence in these traits, since the relative frequency of forced matings tends to be greater in high-predation populations. We found consistent patterns of variation in male genital size and shape in relation to the level of predation, and corresponding patterns of (co)variation in female genital morphology. Together, these data enable us to draw tentative conclusions about the underlying selective pressures causing correlated patterns of divergence in male and female genital traits, which point to a role for sexually antagonistic selection.